1. Field of the Invention
The present invention relates to an image forming apparatus and control method. Particularly, the present invention relates to optimization of the downtime of a discharge mode in which a deteriorated developer is discharged when many images that consume a small amount of toner are output.
2. Description of the Related Art
Recently, electrophotographic image forming apparatuses (for example, copying machine, printer, and FAX machine) are rapidly shifting from monochrome apparatuses to color ones. In a color image forming apparatus, a two-component developer containing a non-magnetic toner and magnetic carrier is used in a developing device within a photosensitive drum. The developing device is replenished with toner, reducing the user running cost. The developer in the developing device is stirred by a stirring screw to charge the toner. The developer is then supplied to a developing sleeve and develops an image on the photosensitive drum.
At this time, if images small in toner consumption amount are output successively, toner is repetitively rubbed and stirred for a long time, and deteriorates. To output high-quality images for a long period, the deteriorated toner needs to be replaced with a new one. Proposals for it have conventionally been made.
Japanese Patent Laid-Open No. 09-034243 describes a discharge mode in which a deteriorated developer is discharged when many images that consume a small amount of toner are printed. In the discharge mode, a toner band is generally formed by superposing all colors on an intermediate transfer belt.
When a toner band is formed by superposing all colors (for example, four colors) on the intermediate transfer belt in the discharge mode, the downtime of the discharge mode itself can be reduced. However, the toner band of the four superposed colors needs to be cleaned without generating a cleaning failure. This requires high cleaning performance for the intermediate transfer belt. If the cleaning performance for the intermediate transfer belt is not high enough to clean the toner band of the four superposed colors at once, the number of toner band colors to be superposed needs to be decreased to two or three. However, a toner band to be formed is created in accordance with the average density of images which were output before. If colors are always superposed in the same combination, the downtime of the system may be prolonged depending on the length of a toner band to be formed.